Sunday, April 16, 2017

Mysterious Cosmic Explosion Puzzles Astronomers

We report on the detection of a remarkable new fast high-energy transient
found in the Chandra Deep Field-South, robustly associated with a faint
(mR=27.5 mag, zph∼2.2) host in the CANDELS survey. The
X-ray event is comprised of 115+12−11 net 0.3-7.0 keV counts, with a
light curve characterised by a ≈100 s rise time, a peak 0.3-10 keV flux
of ≈5×10−12 erg s−1 cm−2, and a power-law decay
time slope of −1.53±0.27. The average spectral slope is
Γ=1.43+0.23−0.13, with no clear spectral variations. The
\hbox{X-ray} and multi-wavelength properties effectively rule out the vast
majority of previously observed high-energy transients. A few theoretical
possibilities remain: an "orphan" X-ray afterglow from an off-axis
short-duration Gamma-ray Burst (GRB) with weak optical emission; a
low-luminosity GRB at high redshift with no prompt emission below ∼20 keV
rest-frame; or a highly beamed Tidal Disruption Event (TDE) involving an
intermediate-mass black hole and a white dwarf with little variability.
However, none of the above scenarios can completely explain all observed
properties. Although large uncertainties exist, the implied rate of such events
is comparable to those of orphan and low-luminosity GRBs as well as rare TDEs,
implying the discovery of an untapped regime for a known transient class, or a
new type of variable phenomena whose nature remains to be determined. See: A New, Faint Population of X-ray Transients